Conventional methods for mounting LED elements to substrates have used wire bonding methods. In a wire bonding method, as illustrated in FIG. 5, a surface having electrodes (a first conductive electrode 104a and a second conductive electrode 102a) of an LED element is faced upwards (face-up), and this LED element is electrically connected to a substrate by using wire bonds (WB) 301a and 301b, and the LED element is bonded to the substrate using a die bonding material 302.
However, in an electrical connection obtained by methods using such a wire bonding, there is a risk that the wire bond might physically break away or detach from the electrodes (the first conductive electrode 104a and the second conductive electrode 102a); more reliable techniques are thus desired. Furthermore, oven curing used in curing processes of the die bonding material 302 increases production time.
As illustrated in FIG. 6, as a method in which wire bonding is not used, the surface having the electrodes (the first conductive electrode 104a and the second conductive electrode 102a) of the LED element have been faced towards the substrate (face-down, flip-chip), and the LED element and the substrate have been electrically connected using a conductive paste 303a and 303b of which silver paste is a typical example.
However, low adhesive strength of the conductive paste 303a and 303b necessitates reinforcement with a sealing resin 304. Furthermore, oven curing used in curing processes of the sealing resin 304 increases production time.
As illustrated in FIG. 7, as a method in which conductive paste is not used, the electrode-side surface of the LED element is faced towards the substrate (face-down, flip-chip), and an anisotropic conductive adhesive comprising an electrically insulating adhesive binder 305 having conductive particles 306 dispersed therein is used to electrically connect and bond the LED element and the substrate. Because bonding processes of anisotropic conductive adhesives are short in duration, productivity is favorable. Moreover, anisotropic conductive adhesives are inexpensive and have excellent properties such as in transparency, adhesiveness, heat tolerance, mechanical strength and electrical insulation.
In recent years, LED elements for flip-chip mounting have been under development. In such LED elements for flip-chip mounting, designs capable of large electrode surface areas are enabled by a passivation layer 105, thus enabling bumpless mounting. Furthermore, providing a reflective layer beneath an emissive layer increases light-extraction efficiency.
As illustrated in FIG. 8, gold-tin eutectic bonding can be used as a method for mounting a flip-chip-mounting LED to a substrate. Gold-eutectic bonding is a method for eutectic bonding comprising forming chip electrodes from a gold-tin alloy 307, applying a flux to a substrate, mounting the chip, and heating to connect chip electrodes to the substrate. However, in such a method using solder connection, yield rates are poor because chip misalignment during heating and flux remaining after cleaning adversely affect reliability. Moreover, advanced mounting techniques are necessary.
As a method in which gold-tin eutectic bonding is not used, as illustrated in FIG. 9, a solder connecting method using a solder paste 303 has been used to create electrical connections between the electrode-side surface of the LED element and the substrate. However, in such solder connection methods, isotropic conduction of the solder paste generates short circuits between p-n junction electrodes, leading to poor yield rates.
As a method in which solder paste is not used, as illustrated in FIG. 10, to electrically connect and bond the LED element and the substrate, as in FIG. 7, an anisotropic conductive adhesive such as an ACF having an electrically insulating binder in which conductive particles 306 are dispersed has been used. In the anisotropic conductive adhesive, the electrically insulating binder fills spaces between p-n junction electrodes. Short circuits are thus unlikely to occur and thereby yield rates are favorable. Furthermore, because bonding processes are short in duration, productivity is favorable.
However, an active layer (junction) 103 of an LED element generates a significant amount of heat in addition to light, and an active layer temperature (Tj=junction temperature) of 100° C. or more reduces light-emission efficiency and lifetime of the LED element. A configuration enabling efficient dissipation of heat from the active layer 103 is therefore required.
In such wire bond mounting as illustrated in FIG. 5, situation of the active layer 103 in an upper portion of the LED element results in inefficient heat conduction towards the substrate side, leading to poor heat dissipation.
Flip-chip mounting as illustrated in FIGS. 6 to 10 in which the active layer 103 is situated on the substrate side enables efficient heat conduction towards the substrate side. As illustrated in FIGS. 6 and 9, in the case of bonding between electrodes with conductive paste 303a and 303b, efficient heat dissipation is enabled; however, connections made using the conductive paste 303a and 303b have poor connection reliability as described above. Furthermore, as illustrated in FIG. 8, in the case of using gold-tin eutectic bonding, connection reliability is poor as described above.
Additionally, as illustrate in FIGS. 7 and 10, by not using the conductive paste 303a and 303b and by flip-chip mounting with an anisotropic conductive adhesive such as an ACF (anisotropic conductive film) or an ACP (anisotropic conductive paste), situation of the active layer 103 near the substrate side leads to efficient heat conduction to the substrate side. Furthermore, high connection reliability is obtainable due to high adhesive strength.